Merge pull request #5977 from j8asic/patch-1

Allow NVCC 12 to compile using C++20 flag

bin/nvcc_wrapper

@@ -338,6 +338,24 @@ do
     std_flag=$corrected_std_flag
     shared_args="$shared_args $std_flag"
     ;;
+  --std=c++20|-std=c++20)
+    if [ -n "$std_flag" ]; then
+      warn_std_flag
+      shared_args=${shared_args/ $std_flag/}
+    fi
+    # NVCC only has C++20 from version 12 on
+    cuda_main_version=$([[ $(${nvcc_compiler} --version) =~ V([0-9]+) ]] && echo ${BASH_REMATCH[1]})
+    if [ ${cuda_main_version} -lt 12 ]; then
+      fallback_std_flag="-std=c++14"
+      # this is hopefully just occurring in a downstream project during CMake feature tests
+      # we really have no choice here but to accept the flag and change  to an accepted C++ standard
+      echo "nvcc_wrapper does not accept standard flags $1 since partial standard flags and standards after C++14 are not supported. nvcc_wrapper will use $fallback_std_flag instead. It is undefined behavior to use this flag. This should only be occurring during CMake configuration."
+      std_flag=$fallback_std_flag
+    else
+      std_flag=$1
+    fi
+    shared_args="$shared_args $std_flag"
+    ;;
   --std=c++17|-std=c++17)
     if [ -n "$std_flag" ]; then
       warn_std_flag

cmake/kokkos_test_cxx_std.cmake

@@ -29,7 +29,11 @@ FUNCTION(kokkos_set_cxx_standard_feature standard)
   ELSEIF(NOT KOKKOS_USE_CXX_EXTENSIONS AND ${STANDARD_NAME})
     MESSAGE(STATUS "Using ${${STANDARD_NAME}} for C++${standard} standard as feature")
     IF (KOKKOS_CXX_COMPILER_ID STREQUAL NVIDIA AND (KOKKOS_CXX_HOST_COMPILER_ID STREQUAL GNU OR KOKKOS_CXX_HOST_COMPILER_ID STREQUAL Clang))
-      SET(SUPPORTED_NVCC_FLAGS "-std=c++17")
+      IF(${KOKKOS_CXX_COMPILER_VERSION} VERSION_LESS 12.0.0)
+        SET(SUPPORTED_NVCC_FLAGS "-std=c++17")
+      ELSE()
+        SET(SUPPORTED_NVCC_FLAGS "-std=c++17" "-std=c++20")
+      ENDIF()
       IF (NOT ${${STANDARD_NAME}} IN_LIST SUPPORTED_NVCC_FLAGS)
         MESSAGE(FATAL_ERROR "CMake wants to use ${${STANDARD_NAME}} which is not supported by NVCC. Using a more recent host compiler or a more recent CMake version might help.")
       ENDIF()